Lower mantle may be rich in silica

Abstract

There have been conflicting claims about the composition of Earth's lower mantle. The latest Brillouin-spectroscopy data suggest that this section of the planet's interior may contain more silica than the upper mantle. See Letter p.90 Earth's upper mantle is apparently depleted of silicon when compared with meteorites, which are thought to represent the material from which the Earth formed. This 'missing silicon' problem has provoked intense debate: it suggests that the deficit might be balanced by silicon in the core, or that the upper mantle is not representative of the entire mantle and that the lower mantle is enriched in silicon. Murakami et al. provide evidence to support the latter case. They made laboratory measurements of sound-wave transmission through silicate perovskite and ferropericlase minerals at pressures and temperatures matching those of the lower mantle, and compared the resulting shear-wave values with seismic data that sample the lower mantle. They conclude that the mineralogical model that provides the best fit is one in which perovskite constitutes greater than 93% of the lower mantle. This suggests that the lower mantle is enriched in silicon in comparison with the upper mantle, consistent with the chondritic Earth model, and that there is limited mass transport between the upper and lower mantle.